1. Field of the Invention
The present invention relates generally to closed-loop feedback systems. More specifically, the invention relates to a controller system configured to adjust the operation of peripheral devices in response to pre-selected operating variables.
2. Background of the Invention
The production of fluids (e.g., water and hydrocarbons) from wells (e.g., coal methane beds and oil wells) involves technologies that vary depending upon the characteristic of the well. While some wells are capable of producing under naturally induced reservoir pressures, more commonly encountered are well facilities which employ some form of an artificial lift production procedure. Certain general characteristics are, however, common to most oil and gas wells. For example, during the life of any producing well, the natural reservoir pressure decreases as gases and liquids are removed from the formation. As the natural downhole pressure of a well decreases, the well bore tends to fill up with liquids, such as oil and water, which block the flow of the formation gas into the borehole and reduce the output production from the well in the case of a gas well and comprise the production fluids themselves in the case of an oil well. In such wells, it is also conventional to periodically remove the accumulated liquids by artificial lift techniques which include plunger lift devices, gas lift devices and downhole pumps. In the case of oil wells within which the natural pressure is decreased to the point that oil does not spontaneously flow to the surface due to natural downhole pressures, fluid production may be maintained by artificial lift methods such as downhole pumps and by gas injection lift techniques. In addition, certain wells are frequently stimulated into increased production by secondary recovery techniques such as the injection of water and/or gas into the formation to maintain reservoir pressure and to cause a flow of fluids from the formation into the well bore.
With regard to downhole pumps, some degree of flexibility is needed in operating the pump as operating conditions change. For example, it is often necessary to adjust the rate of fluid flow through the flow line in order to maintain a desired head pressure. The desired head pressure is determined according to the need to prevent gas from entering the pump in addition to maintaining fluid flow through the pump. Failure to control the head pressure can result in conditions that adversely effect the motor and/or the pump. For example, common occurrences in down hole pumping include “gas lock,” pump plugging, high motor voltage spikes, high or low motor current and other failure modes. Left unattended, these conditions can cause damage to the pump and/or motor.
One conventional solution to common operating problems is to use a Variable Speed Drive (VSD) to control the speed of the motor driving the pump. VSDs affect the motor speed by changing the frequency of the input signal to the motor. Increasing the frequency results in increased motor speed while decreasing the frequency decreases the motor speed. The magnitude of the speed adjustment is determined by monitoring a pressure sensor mounted on the pump. The pressure sensor measures the head pressure and transmits the pressure values back to a computer where the pressure value is compared to a predetermined target value (which may be stored in a memory device). If the measured pressure value is different from the target value, then the VSD operates to change the motor speed in order to equalize the head pressure with the target pressure. In this manner, the motor speed is periodically changed in response to continual head pressure measurements and comparisons.
Despite their effectiveness, the viability of VSDs is hampered by significant adverse effects that occur during their operation. One adverse effect is the introduction of harmonics. Harmonics are sinusoidal voltages or currents having frequencies that are whole multiples of the frequency at which the supply system is designed to operate (e.g., 50 Hz or 60 Hz). The harmonics are generated by switching the transistors that are part of the VSD. Harmonics are undesirable because they can cause damage to peripheral devices (e.g., household appliances such as televisions, microwaves, clocks and the like) that are serviced by the power company supplying power to the VSD. As a result, some power companies have placed restrictions on the use of VSDs.
In addition to the damage caused to peripheral devices, the pump motor and associated power cable may themselves be damaged. Specifically, the high peak-to-peak voltage spikes caused by switching the VSD transistors increases the motor temperature and can damage the motor power transmission cable (due to the large difference between the spike voltage and the insulation value of the cable). As a result, the chance for premature equipment failure is increased.
Therefore, there exists a need for a control system that allows for the operation of pumps and other devices without the shortcomings of the prior art.